1. Field of the Invention
This invention relates to a projection lens apparatus suitable for use in a projective display apparatus for enlarging and projecting pictures of a television receiver.
Such projection lens apparatus for the projective display apparatus should have a larger aperture ratio and a larger angle of view, the larger aperture ratio being necessary to obtain a bright picture by efficiently focusing the luminous flux generated from the cathode-ray tube, the larger angle of view being for achieving a larger projection magnification in a smaller projection distance so as to miniaturize the apparatus.
2. Description of the Prior Art
Conventionally, the kind of lens has employed a glass lens, which has remarkably increased the weight as well as increased the aperture following enlargement of aperture ratio, resulting in the problem created in the construction of the display apparatus and the manufacturing cost. On the contrary, it is possible to reduce the number of lenses while still achieving the effect of the aspherical surface thereof to thereby reduce the weight of the apparatus, but the glass lens leads to an extremely high manufacturing cost because its manufacturing technique is difficult if it is to sufficiently meet the aforesaid requirement.
Recently, projection lenses of plastic material, with the above technical characteristics, have come out. The three-tube system projective display apparatus, which uses monochromatic cathode-ray tubes of green, blue and red, mostly does not require correction of the chromatic aberration because of the relatively narrow width of the emission spectrum of each cathode-ray tube, so that the characteristics of the plastic lens, such as light weight and easy production of the aspherical surface, may make the lens usuable in as a projection lens for the projective display apparatus.
Such projection lens apparatus has been well-known as described in the Japanese Patent Laid-Open No. Sho 55-124114, which has a simple construction using the plastic lens with aspherical surface to achieve the larger aperture ratio as well as using three lenses. Hence, the apparatus is quite advantageous as to the manufacturing cost, but has several problems as follows:
Firstly, the overall lens elements are of plastic so that they have a temperature characteristic which causes a problem, namely, since plastic has a value of the temperature dependency of refractive index which is larger by about two units than that of glass, a drift of the focal position due to the temperature change in the ambient atmosphere, which not been a problem at all with glass lenses, becomes substantial so that when the temperature at the location of its projective display apparatus fluctuates substantially, for example in summer and winter, the focus occasionally shifts so that the apparatus is of no practical use.
Secondly, the second lens has a larger center thickness which will generate a larger thermal gradient at the center and periphery of lens in the heat cycle during the molding of the plastic lens, which may make it difficult to maintain the uniformity of lens.
Thirdly, the overall length of the lenses is larger by about 1.8f from the vertex of the first lens to that of the third lens, which leads to difficulty in enlarging the angle of view and making the apparatus compact.